In recent years, fluid separation technologies using various separation membranes such as a gas separation membrane, a reverse osmosis membrane, a nanofiltration membrane, an ultrafiltration membrane, a microfiltration membrane, and the like have received attention as a fine and energy saving process, and the application of such technologies to treatment of various fluids has been promoted. For example, in a reverse osmosis separation process using a reverse osmosis membrane, a solution containing a solute such as salt is allowed to permeate the reverse osmosis membrane at a pressure above the osmotic pressure of the solution, thereby yielding a liquid that has a reduced concentration of the solute such as salt. For example, the process is widely applied for desalination of seawater and brine water, production of ultrapure water, concentration and recovery of a valuable material, and the like.
For efficient operation of such separation membrane, contamination of the membrane surface, which is called fouling, is the most significant problem. This is a process in which impurities contained in a stream-to-be-treated deposit or adsorb onto a membrane surface and a separation membrane channel, thereby degrading the performance of the separation membrane. To prevent the fouling, measures are applied, including pretreatment by filtration and the like to preliminarily remove the impurities, and generation of turbulence in separation membrane module channels to reduce the tendency to deposit the impurities. When a membrane is contaminated despite such measures, a method for rejuvenating the membrane such as washing the membrane with chemicals is applied. However, if the pretreatment is insufficient and/or a large amount of impurities are contained, penetration of materials that cause fouling into the separation membrane is often not prevented. If possible, it is preferred not to wash a membrane with chemicals, because, for example, the washing process usually requires shutdown and the cost of the chemicals, as well as the chemicals degrade separation membranes. Thus, procedures called physical washing are often applied before the need for washing with chemicals, the procedures including a flushing process in which raw-water-to-be-treated or permeate water is fed at the raw water side of a membrane at a high flux, a backwashing process in which pressure is applied onto the permeate side of a membrane to move the permeate water backward toward the raw water side of the membrane to float and remove deposited foulants.
A separation membrane has various configurations such as flat sheet, tubular, and hollow fiber configurations. In the case of a flat sheet membrane, the membrane is often used in the form of a spiral-wound membrane element. As illustrated in Patent Document 1, for example, a conventional spiral-wound membrane element includes one or more laminates of a separation membrane with sealed edges to ensure that a feed stream and a permeate stream do not mix with each other, a feed side channel spacer, and a permeate side channel spacer, the laminates being wound in a spiral around a perforated center tube, and an anti-telescoping plate attached to both ends of the wound laminates.
In such separation membrane element, a stream-to-be-treated is fed at one of the end surfaces and then flows along the feed side channel spacer to pass part of the ingredients (for example, water in the case of desalination of seawater) through the separation membrane, thereby separating the stream-to-be-treated. Then, the ingredient that has passed through the separation membrane (permeate water) flows along the permeate side channel spacer into the center tube through holes in a side of the tube, and then passes through the center tube to be collected as a permeate stream. On the other hand, the treated water containing a high concentration of a non-permeate ingredient (salt in the case of desalination of seawater) is collected from the other end surface of the separation membrane element as concentrated water. Such spiral-wound membrane element has an advantage of tending to reduce channeling due to uniform distribution of channels for a stream-to-be-treated, but if pretreatment is insufficient, there is a problem of tending to deposit foulants on the end surface to which a stream-to-be-treated is fed.
Particularly, in the spiral-wound membrane element, usually one or more separation membrane elements are often disposed in series in a single pressure vessel. In this case, fouling as described above dominantly occurs in, especially, the front end region of a first separation membrane element. And, in the case of desalination of seawater, due to osmotic pressure, the first element exhibits high permeation flux, because the element receives less concentrate stream, and tends to deposit foulants on the membrane surface, thereby promoting fouling. On the other hand, as the solute is concentrated, and then osmotic pressure increases, the last separation membrane element has lower permeation flux and thus is less likely to cause fouling. On the contrary, a later separation membrane element has the property of receiving a stream-to-be-treated at a lower flow rate due to permeation through a prior element, and thus producing reduced flushing effect on the membrane surface, so foulants deposited on the membrane surface are difficult to remove from the membrane surface. To maintain the flow rate of a stream-to-be-treated, a separation device often uses a configuration in which a plurality of separation membrane unit components 8a, 8b, and 8c are organized in a tree structure, as illustrated in FIG. 18, to decrease the number of separation membrane unit components 8c in the later stage, for matching with the reduced flow rate, thereby maintaining the flow rate of the stream fed to the separation membrane unit. In FIG. 18, 7a represents a valve, 26 represents a stream-to-be-treated, 27 represents a permeate stream, and 28 represents a concentrate stream.
In view of the foregoing problems and characteristics, for example, Patent Document 1 proposes a method of periodically feeding permeate water into a concentrated water outlet to flush the permeate water in the opposite direction to the flow of a stream-to-be-treated, and Patent Document 2 proposes a method of switching the flow between forward and reverse directions on the stream-to-be-treated side for flushing. These methods allow removal of foulants depositing on the ends of a separation membrane element, and, when stream flows in the forward direction, allow removal of foulants depositing on later separation membrane elements, which tend to produce reduced flushing effect on the membrane surface. Patent Document 3 and Patent Document 5 propose a method of reversing the flow direction of a stream-to-be-treated during operation to remove foulants depositing on the surface of a separation membrane, with little downtime. As illustrated in Patent Document 4, such method is also applied to a plurality of separation membrane unit components organized in a tree structure to allow reversal of the flow direction of each stream-to-be-treated.
Although these methods are applicable to a system that has a substantially similar structure from the feed water inlet to the concentrated water outlet so that the performance does not change even when a flow direction is reversed, as in a hollow fiber membrane module, the spiral-wound membrane element as described above uses a sealing material that effectively functions only in a single flow direction when a separation membrane element is disposed in a vessel, and thus provides poor sealing when the flow direction is reversed. Although such poor sealing is acceptable in backwashing, treatment efficiency decreases due to, for example, energy loss, in an operation in which a stream-to-be-treated flows in the reverse direction for a long period.